The stock market loves Apple (NSDQ: AAPL), and so, it seems, do consumers falling over themselves to get news about the latest iPhone. But it appears that Apple device usage may be a different story. Millennial Media, one of the bigger mobile ad networks, notes that ad impression growth, on its network, for iOS devices, was flat in the last month, in contrast with Android and Windows, which each went up by 48 percent.

Unusually, that puts Apple’s iOS operating system into the same category as Symbian, which also saw flat growth this month. Ad impressions measure how many times an ad gets called up through an app or another piece of mobile content on the mobile internet.

The figures, from Millennial’s August Mobile Mix report, note that Apple is still the single-most popular OEM on its network, accounting for just over 23 percent of all impressions; the iPhone is also the most popular device, with just over 13 percent of all ad impressions.

But after that, it becomes pretty much an Android story. Android device makers HTC, Samsung, Motorola (NYSE: MMI) and LG (SEO: 066570), Huawei, Kyocera and SonyEricsson (NSDQ: ERIC) taking up seven of the top-ten on the OEM list. (RIM (NSDQ: RIMM) holds out at number five, and Nokia (NYSE: NOK) ranked eighth.) In device-specific rankings, Android accounted for six of the top-ten devices:

HTC, on the heels of new product launches, has reclaimed the spot as the number-two OEM after Apple in terms of ad impressions. That slot had been occupied for the last several months by Samsung.

Android now accounts for 54 percent of all ad impressions on the Millennial network, with Apple in an increasingly trailing second position at 28 percent. Android’s dominance on Millennial’s network is a good example of how strong an OS can become when it works on multiple OEMs’ devices.

You would think that this would make the platform hugely appealing to those in the mobile advertising world looking to reach the biggest audience possible.

But interestingly, when it comes to ad spend, the two platforms are much closer than their impression shares would suggest. This month, Android accounted for 49 percent of ad spend, while iOS accounted for 41 percent of ad spend. The month before, in July, the gap was slightly smaller, with Android at 48 percent and iOS at 43 percent. A sign that Android is gradually pulling away?

The other side of the story, of course, is that together Apple and Android are still dominating over every other platform when it comes to mobile ad spend on the network.

Content. Largely a matter of single place swapping if you compare consumer preferences in August 2011 compared to the same month a year ago. Games are still on top, with Heath and Fitness the only new category entering the top 10.